Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGVariation of hull resistance at moderatespeeds of any well-designed ship is approximatelyproportional to the square of the speed.The power required to propel a ship is proportionalto the product of the hull resistance andspeed. Therefore, it follows that under steadyrunning conditions, the power required to drivea ship is approximately proportional to the cubeof propeller speed. While this relationship isnot exact enough for actual design, it does serveas a useful guide for operating the propellingplant.Since the power required to drive a ship isapproximately proportional to the cube of thepropeller speed, 50 percent of full power willdrive a ship at about 79.4 percent of the maximumspeed attainable when full power is usedfor propulsion, and only 12.5percent of full poweris needed for about 50 percent of maximum speed.The relation of speed, torque, and horsepowerto ship's resistance and propeller speedunder steady running conditions can be expressedin the following equations:whereS = k.X (rpm)v2T = k_ x (rpm)shp __27rk2_ X (rpm)33,000S = ship's speed, in knotsT = torque required to turn propeller, infoot-poundsshp = shaft horsepowerrpm = propeller revolutions per minutek. , k„ 3 proportionality factorsThe proportionalityconditions such as displacement, trim, conditionof hull and propeller with respect to fouling,depth of water, sea and wind conditions, and theposition of the ship. Conditions that increasethe resistance of the ship to motion cause kj tobe smaller and k2 to be larger.In a smooth sea, the proportionality factorsk^ and k2 can be considered as being reasonablyconstant. In rough seas, however, a ship is subjectedto varying degrees of immersion and wavefactors depend on manyimpact which cause these factors to fluctuateover a considerable range. It is to be expected,therefore, that peak loads in excess of the loadsrequired in smooth seas will be imposed on thepropulsion plant to maintain the ship's ratedspeed. Thus, propulsion plants are designed withsufficient reserve power to handle the fluctuatingloads that must be expected.There is no simple relationship for determiningthe power required to reverse the propellerwhen the ship is moving ahead or the power requiredto turn the propeller ahead when the shipis moving astern. To meet Navy requirements,a ship must be able to reverse from full speedahead to full speed astern within a prescribedperiod of time; the propulsion plant of any shipmust be designed to furnish sufficient power formeeting the reversing specifications.PROPELLERSThe propelling device most commonly usedfor naval ships is the screw propeller, so calledbecause it advances through the water in somewhatthe same way that a screw advances throughwood or a bolt advances when it is screwed intoa nut. With the screw propeller, as with a screw,the axial distance advanced with each completerevolution is known as the pitch. The path of advanceof each propeller blade section is approximatelyhelicoidal.There is, however, a difference between theway a screw propeller advances and the way abolt advances in a nut. Since water is not a solidmedium, the propeller slips or skids; hence theactual distance advanced in one complete revolutionis less than the theoretical advance for onecomplete revolution. The difference between thetheoretical and the actual advance per revolutionis called the slip . Slip is usually expressed asa ratio of the theoretical advance per revolution(or, in other words, the pitch ) and the actual advanceper revolution. Thus,whereSlip ratioE = shaft rpm x pitch = engineminutedistance perA = actual distance advanced per minuteScrew propellers may be broadly classifiedas fixed pitch propellers or controllable pitchpropellers. The pitch of a fixed pitch propeller88